Induction-motor.



A. W. FORBES.

INDUCTION MOTOR. APPL1CAT10N HLED 1511.21. 1913.

v2l,-1,83,548. Famed May-16, 1916.

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Patented May16,1916.

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A. i1/FORBES.

INDUCTION MOTOR..

APPLICATION FILED'FEBY 21. 19x31 y; if@

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INDUCTION MOTOR.

APPUCATIQN FILED FEB. 2l. 19:3.

if@ 83,548- Patented May 16, 1916.

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fafa/a 5ans Ammer A. W. FORBES.

INDUCTION MOTOR.

APPLICATION FILED FEILZI, IsIa.

' Patented May16,1916.

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mnucnoN Moros.

APPLICATION FILED FEB' 21| 1913. 1,1 83,5541@i Patented May 16, 1916.

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A. W. FORBES.

INDUCTION MOTOR. I

APPLICATION FILED FEB,2|.1913.

1,1 83548 Patented May16, 1916.

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inermi) sTA'rEs ALLAN W. FORBES, OF WORCESTER, MASSACHUSETICS, ASSGNGR T0 GRQG'KEEJ- WHEELER COMPANY, 0F AMPERE, NEW JERSEY, A CGRPORATQN Gl? NEW? JERSEY.

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To all whom t may concern Be it known that I, ALLAN XV. Forums, a citizen of the United States, and resident of V'orcester, in the county of lVorcester and State ot Massachusetts, have invented certain new and useful Improvements in Induction-Motors, of which the Jfollowing)r is a specification. f

My invention relates rent induction motors, or polyphase.

` The object ot' my invention is to construct such motors so that they will start and open ate at low speeds as well as high speeds in a satisfactory manner.

ln carrying out my invention l change the number ot poles produced by the stator or primary winding` ot the motor in order to alternating cnreither single-phase to obtain diiterent speeds and i construct the system ot conductors that form the secondary circuit so that its resistance with re spect, to the currents induced in it is relatively greater for the lower speed or speeds.

It is well known that the squirrel cage construction is a very desirable torni ot rotor for induction motors because ot its mechanical as well as electrical simplicity and sturdiness. lt comprises an iron core carrying anumber of inductors or loneiti'e dinal conducting' bars connected together at both ends to obtain a closed secoiulaiv eircuit of low resistance. This low resistance is an advantageous condition when the rotor is operating at or near tull speed. On the other hand. for starting an induction motor -oiffor running; it at low speed or speeds less than those approximating' the maximum. it is 'usually desirable that the secondary resistance should be relatively greater than for high speed operation. llereto'torc this increased resistance has not been readily obtainable without sacrificingV lthe simplicity and freedom from moving electrical coutacts which constitute .special merits oil the suuirrel cage, rotor. One method tor obtaining low speeds or multispeeds consists in altering the, pole pitch or distance between the magnetic poles toi-med by the alternating current or currents in the primarv wind in sr ot the stator. For example. an eightpole machine. that runs nearly 900 l2. l. M. with a trequencv/ of (30, will run at about one-halt that spec( o' about. l-SO ll. l. ht it the prin'inr)Y winding is so larranged and. eonnected that it torins sixteen poles in- Specification of Letters Patent.

Application filed February 21, i913.

stead ot eight, at the same time the pole pitch ot course becomes one-halt as great as before. lil'ith a rotor ot' the ordinary squirrel cage type, the seconcary resistance is actually when it is starting or running;` at lower speed with the reduced pole pitch, because the paths oli' the induced secondary with the smaller pole pitch.

i''v mean have invented the secondary resistance an induction motor may be designed be as high as desired tor starting` or operai; Y at low speeds: at the saine tine said sec ondary resistance may be desile'i'ied to he as low as desired when the. machine is runnin at or near its 'full speed. These advantage, ous conditions are obtained without requin in;r any moving parts or contacts in or on the rotor. To accomplish these results construct and combine the conducting' bars and end connections which torni the secondary circuit et an incuction motor. so that the induced currents are caused to pursue high resistance l low speeds and are atforded low resistance paths tor hi f fh speeds.

The accompanying drawings set orth the nature ot' my invention and illustrate some of the forms o't construction that may be en'iployed in practising it. i

liie'ure l represents a portion of the rotor bars of an induction motor laid out in a plane to show the relative pole pitch tor high,4 intermediate and low speeds, also the end connector pitch. Fig. .2 shows a torni ot end connector by which the rotor be joined to form a secondary winding' aecordinp; to my invention. Fig. 3 represents a portion ot the surface oit a rotor. includ ing' the core. rotor hars and end connectors s or the ':onstruction which l paths for startingor tor.

bars may Sti arranged in :ucordauce with my intention.

be made as high as desired for tartine' or a es . Ilhigher resistance suitable for 2 Licence for low speeds. Fig. 7 illustrates a form of my end connector adapted to be used in the Winding represented in Fig. 6. In Fig. 8 is shown a small auxiliary end connector adapted to produce the same result of higher resistance for low speeds. Figs. 9 and 10 represent a modified form of my construction of bars and end connectors, correspond- .ing to that shown in Figs. 4, 4a, 5, and 5,

but having connectors perpendicular to the bars instead of oblique thereto. Figs. l1

y and`12 also represent modified arrangements o'f'bars and end connectors according to my Invention. Fig. 13 shows a secondary circuit of lov.T resistance suitable for high speeds and Fig. 14 a secondary circuit of relatively starting or low speeds.

In Fig. 1 the vertical lines marked Ito.

25 respectively, represent a portion of the inductors' or longitudinal conducting bars of the rotor of an induction motor. In the ordinary squirrel cage rotor these bars are all connected together at both ends by what are called end rings or equivalent construction. For the purposes of my invention, 'these bars-are connected in a special manner.

. For example bar 1 is connected by a confmately equal to this as also shown in Fig. 1.

ductoi' of low resistance to bar 13. Similarly bar 2 is connected to bar 14 and so on. In this oase the conductor spans 12 bars,l and this distance I call the end connector pitch, as shown in Fig. 1o The pole pitch or distance between adjacent poles produced by the primary winding which corresponds to v:high speed may be'made equal or approxiend connector pitch, The pole pitch for lower speeds is less, being usually about onehalf as much for onc-lialf speed and so on,

` as represented.

form of end connector that may be emi l ployed in carrying outiny invention is illustrated in Fig. 2.

Its length is L L and part of itsoutline is indicated by dotted lines because lit is overlapped by the next connector the outline of which is shown in solid lines. The three inductors or bars A,

1 and 2 are connected to one end of the first connector, the other end of which connects With the bars 12, 13 and 14, as represented. The end connector pitch is its average span which is from bar 1 to bar 13, that is, twelve bars,'as shown. Similarly the next connector unites the bars 2, 3 and 4 at one end to the bars 14, 15 and 16 at its other end, the average span or connector pitch being twelve bars, as before. The other bars are electrically connected in a corresponding manner by end connectors.

In Fig. 4 the bars marked A, 1, 2, etc., to 26 inclusive, are shown diagraininatically with their connections at one end of the rotor. The bar 1 is connected by the end connector C D to bar 13, as shown, the black dots at the intersection ot' two lines indicating electrical connection. at that point. It is important vto note that the end connector C D is connected at one end not only to bar 1 but also to the two adjacent bars A and 2 and at the other end it connects with bars 12 and 14 as well as bar 13. In other words, a group of three consecutive bars are connected to another group of three consecutive bars, the average distance between the two groups being called the end connector pitch which in this case spans twelve bars. The next end connector J d joins the bars 2, 3 and 4 to the bars 14, 15 and 16, the pitch being the average distance, that is from the point K where it connects with the bar 3 to the point al where it connects with the bar 15, a span of twelve bars, as before. If now the primary winding be arranged and connected to produce such a number of inagnetic poles' that the pole pitch or distance between them is about equal to the connector pitch, it is evident that the current induced upward in bar 1 for example, will flow di rectly to bar 13 and downward therein, as indicated by the arrows. The resistance in the path of this current may be .made as low as desired by having the end connector C D of the proper material and cross section. With the arrangement shown in Fig. 4, the actual fact is that three bars A, 1 and 2 in which maximum current tends to be genen ated upward, are connected by three end conductors, that is-C Dv and those on eithci` side of it) to the three bars 12, 13 and. 14 in which maximum curi-ent tends to be generated downward. Hence strong currents may be readily set up in this low resistance path. At the other end of the rotor these currents flow through corresponding connectors to the bars 24, 25 and 26 in which maximum current tends to be generated upward, and so on all around the armature, the total nuinber of bars being approximately a multiple of 12 but preferably a prime number. In order to bring outclearly the active bars and connectors with the direction of the currents therein, these are separated fromthe others and shown by themselves in Fig. 4'l 'The currents flowing downward in the three bars l2, 13 and 14 are brought from both directions by the three end connectors C D,

J CZ, etc., on the left and the three end connectors V Y, F and Z W on theright. Figs. 4 and' /lshow only the bars and connectors at one end of the rotor, a corresponding arrangement being made at the other end.

In order to run the rotor at lower speed, the primary winding is so connected that the number of poles produced by it is iiicreased and the pole pitch reduced accordingly. For example, if the pole pitch is made one-half as much as before, we have the Conditions represented in Figs. 5 and 5".

meente The tendency tor strong currents to flow from har 1 to har t8 no longer exists hecausc the F. lill. is generated upward in both liars and one opposes the o tllith. the pole pitch reduced to onenhan its former value the tendency is `for current low np- 'Ward in baril, downward in loer 7, and so on, as represented, 'the pitch new heine' six bars instead or? twelve hars. The path open to these currents is the zig-zag; route via, H, J, ll, ll', P and Q, as indicated hy the arrows in Figs. and 5a. This path is of comparatively high resistance lceccuse it has only the vcrinss section of one connector. There is also other path at the top of has, from. 2 to i; and then to ll, as shown, through which current flows. ln Figs. 5 and these paths conhined, however, oi? higher resistance than the' three direct connectors of Figs. l and da. llforeover the zig-neg paths are also open to 'the currents in Fig. 4, as indicated ley arrows therein. Hence the currents have two zig-zag paths and three direct paths, equivalent to five end connectors in Fig. 4, whereas they have only two zig-zag 'paths equivalent to two connectors in 5 and The result is that the secondary resistance is relatively much higher for the smeller pole pitch and lower speed; represented hy Figs. 5 and 5a than it is for the larger pole pitch and higher speed, represented by Figs. 4f and 4a. As already stated these conditions are advantageous. lf it is desired to increase still further the secondary resistance for starting or for low speeds,

. H'the following modifications oi my invention,

shown in Figs. 6, 7 and 8 may oe employed:

1n Fig. 6, a portion of the rotor hars and end connectors are represented. The ends of the connectors are reduced in thickness, as shown, so that the portion that joins a. group of three adjacent bars. together is thinner than the rest ot' the connector. For example, the end portion J, M, of

the connector that electrically unites the rotor hars 2, 3 and 4 is shown much thinner than the rest of the connector. The other connectors are made in a similar manner. The result of employingr this construction is to increase the secondary resistance 'for starting or low speeds with small pole pitch, `which in this case represented to he from har 1 to bar 7, that is six hars. The secondary circuit may he made to have any desired increase in resistance for starting,r or low speeds by reducing the thickness of the ends of the connectors, such as J, M, or N, l), etc. lilith this construction the zig-nap; paths of 'the currents oorrespmiding to the small pole represented in Fig. also in Figs. 5 and 5 has relatively l. much higher resistance than the directpaths of the currents with pole .pitch nearly equal to tho eno connector pitch, as in Figs. d and t,

:in the former case, thc secondary current flows from bar 1 to har 7 which distance equals the low pole pitch, that is six bars, and has to traverse six sections of end connector ot reduced thickness, as indicated by the arrows, whereas, with the full pole pitch about equal tothe end connector pitch, the secondary current flows directly from bar 1 to bar 13, (the full pitch of twelve bars) through the portion of the connector that has greater thickness and therefore less resistance, the ratio depending upon the relative thickness ci the diiierent portions of the connectors. To he sure one-half of the hars, for example, liars 1, 8. etc., are not connecte hy the thicker portions of the con'- nectors, and the current from them must' pass through one section olf the thinner part of the connector, shown in Fig". (3. But the resistance oii'-tliis one section is only one. 85 sixth of the resistance for the current with the smaller pole pitch. it' desired, the connectors may be made 1with full vthickness except the extreme end section. For eX- ainple, only the part d, l, is thin, the part K, M, being' made as thick as the rest of the connector. ln this way each har is connected to a connector of full thickness.

n Fig. T another iorm of end connector is shown which muy he designed to give any desired degree oi' increase in secondary resistance for/'starting or .low speeds. The ends oi the vconnector are reduced iu cross section hy cutting' away somo ot' their depth, as rcprescntedwit u, 7, l, or it may he fur-i 100 ther cutgaway, as indicated hy dotted lines. 0i' course the ends of the connectors may at the saine time he reduced in thickness, as in Fig. G, in order to increase the resistance still further. similar result may bc ob- 105 tained by the use of a small auxiliary connector, such as that illustrated in Fig. S. in this case, the main end connector could be made of any suitable cross section to ,give the proper mechanical strength and rela- 110 tively low electrical resistance. This connector would extcml from bar 1 to har 13 in Fig. 7, and han A would he connected to bar 1 hy the comparatively high resistance auxiliary connector shown in Fig. S. saine for hars 13 and 14. lt so desired, the/ main connector may extend only from bar 2 to har 12 and an auxiliary connector may span the three hars A, 1 and 2, also another auxiliary connector for the bars 12, 13 and'.120 14. '.lhese auxiliary connectors may have any required resistance by making them of l the proper thinness, depth and material.

The particular type of end connectors illustrated in Figs. 4, 4, 5, 5 and 6 are 125 oblique to the rotor bars, beingT in that respert similar to those set forth in U. S. latrnt No. ldllh-l granted to (inno Dunn. lt is not essential, however, to employ this emistruction in order to practise my inveir 13G The tion. Figs.` 9 and 10 illustrate the application of my invention by means of end connectors that are at right angles to the rotor bars. In general, however, the arrangement therein shown is similar to that of Figs. 4, 5, 5 and G already described. Fig. 3 1s a general view of a portion of a rotor having perpendicular end connectors arranged according 'to my invention. Only part ot the end connectors are shown at the right hand end ci' the rotor.

It is usually desirable to have a vprime number o t' rotor bars with respect to the number of poles. In order to simplify the drawings, however, simple numerical relations have been adopted. By adding or subtracting one bar or some other suitable number of bars7 any desired number, either prime or otherwise, may be obtained. In Fig. 11, `for example, an additional bar iS introduced, and an additional end connector extending from the point marked to the point marl-,red 15 is also introduced.

In order to simplify the drawings and make themu'eadily comparable, in most cases I have illustrated the same number of rotor bars, also the same pole pitch as ivell as end connector pitch. but my invention may be practised vvin many different numerical relations. In Fig. 12, for example, the connector extends from the bar A to the bar 16 and connects the four bars A., 1, 2 and at one end to the four l bars 13, 1.4-, 15 and 1G at its other end.

W ith this arrangement the increase in resistance et the end connectors indicated in Figs. 6, 7 and 8, may be applies, to all lthree of the sections 33 to 3G or to any one or two of them. It is also ay fact in Fig. 12 that the average distance or pitch between the two groups of bars is thirteen instead of twelve. Various other nrunbers of bars, pole pitch., end connector pitch. grouping of bars, etc., may be employed, if desiree.

In Fig. 13 is represented a secondary circuit of relatively low resistance adapted to operation of the motor at the higher speeds with the larger pole pitch. In Fig. 14 is shown a secondary circuit of relatively high resistance suitable for a motor that is starting or runninor at the lower speeds vsith the smaller pole pitch.

I prefer to malte the end connectors alike so that they are interchangeable for the sake of cheapness of construction and repair, but it is obvious that one or more of Ithem may have a different size or shape to `\su1t a particular number or arrangement of bars or for other special reasons.

I have shown and described a variation in pole pitch in the ratio of one to two, that is, a pole pitch for starting and low speeds which is one-lialie as large as that employed for higher speeds. It desired a still greater .ratio et' pole pitch may be adopted, for en ample, oneto three or one to four. It is also a fact that intermediate ratios of pole pitch may also be employed. For example, a pole pitch that is three-quarters of the maximum may be used as indicated in Fig. 1

In this specification I have referred in a generic sense to the rotor which I employ as being o't the squirrel cage type. Ordinarily this term is applied to rotors in which the bars are all connected together and short circuited by end rings or similar construction. The `drawings and description which set forth my invention show clearly that my form of rotor differs from the ordinary type, because the bars are connected in a particular manner and not simply short circuited. In some cases it may be desirable to make use of my connectors at only one end of the rotor; ordinary end rings or other connections being employed at the other end of the rotor.

In Fig. 2 the rotor bars 5, 6, 7, 8, 9, 10 and 11 are shown separated from the two end connectors by a considerable distance. This is not necessary because a slight separation would be suilicient. In fact it would do no harm if the end connectors should bend outward and touch the bars at high speeds as it would only result in reducing the secondary resistance7 which should be low at that time. would serve to prevent the end connectors from bending outward too much by centrifugal force.

I cla-im:

1. In an induction motor, the combination with the rotor bars of connectors each of 'which is electrically connected at both ends to a plurality of said bars and spans several bars with which it is not directly connected, one end of each bar being connected to at least two of said connectors and the average distance between the tivo groups of con nccted bars being approximately equal to the pole pitch.

2. In an induction motor, the combination with the rotor bars of connectors each of which is electrically connected at both ends to a plurality ot said bars and spans several bars with which it is not in contact` one` end of each bar being connected to at least ztivo of said connectors and the average distance between the two groups of connected bars being approximately equal to the pole pitch.

3. In an induction motor the combination with a primary element of variable pole 'pitch of a secondary winding comprising inductors and elements. each' of which electrically connects at both ends with a plurality of inductors and spans several in ductors with which it docs not contact. one end el cach bar being connected to at least two of said connectors, th 1. average distance between the two groups of inductors beine In this case the bars approximately equal to the larger pole pitch.

4. In an induction motor having a stator of variable pole pitch the combination with the rotor bars otelcmcnts cach of which electrically connects at both ends with a plurality of said bars and spans several bars with which it is not in direct electrical connection, one end of cach bar being con-l nected to at least two ot' said connectors, the average distance between thc two sets of connected bars bcingjr approximately equal to the llarger pole pitch.

5. In an induction motor, having a stator of variable pole pitch the combination with the rotor bars ot' elements each of which electrically connects at both ends with a ALLAN W. FORBES.

at lVorcester in the county of Vitnesses Haaanc'r'r M. Formes, lVILLIAM T. FORBES.

and State of Massachusetts, this' 

